Deep-sea Gas-liquid Multiphase Riser VIV And Fatigue Damage Analysis

The deep-sea gas-liquid multiphase riser works in a complex environment where the riser suffers waves,currents environmental loads and top tension,self-weight and buoyancy loads,its mechanical analysis is complex.When the currents go through the riser(elongated structure),the vortex generates on both sides.When the vortex's frequency gets close to the riser's natural frequency,the riser's coupling resonance occurs,the vibration amplitude increases sharply.If things go on like this,the fatigue damage will occur,the oil platforms will endanger.This article introduces linear wave theory,Stokes wave theory and the Morison equation which are used to analyze the riser's mechanical properties.After choosing the right wave theory and hydrodynamic coefficients,it's able to calculate the total wave force of riser's unit height by Morison equation.Based on the loads of riser,the direct integ ration method is used to solve riser's equations of motion,its response characteristics can be analyzed.ABAQUS is applied to calculate and analyze riser's models response when the riser's boundary is different.On this basis,the riser's inherent frequencies in different environments and characteristics were calculated.When the frequency's order is lower,the gap between the frequencies of riser's dry and wet modes is bigger;however,when the model increases,the gap between the two modes gets smaller.The natural frequency of the riser reduces when riser's length,thickness and outer diameter increase.But it increases when the top tension increases.On the basis of the third boundary condition,by changing the current velocity,the riser's “mismatch” and “lock” phenomenon were found.Then the FLUENT was used to study the flow past circular cylinders.The vibration characteristics of a naked riser and a riser equipped with spiral plates were also studied,the second riser has a larger screw-side flow rate.To a certain extent,the VIV was inhibited.Finally,based on the stress-time curve obtained by ABAQUS,the Smax can determine whether the riser is safe.The rain flow counting method and different S-N curves were used to calculate the deep-sea riser's fatigue life.It is found the cross-section at the top of the riser is the most dangerous section.For different S-N curves of fatigue damage,the D curve of DNV standard is the most stringent.